In tracheotomy patients, because inhaled air enters the trachea directly it is very important for health reasons and for the patient to be able to breath comfortably that the inhaled air is at substantially the same temperature and contains the same quantities of moisture and dust as if it had reached the trachea after passing through the upper airway (nostrils, nose, pharynx and larynx), meaning a temperature approaching 32.degree. C. with a moisture content approaching saturation at the temperature of this air and substantially free of dust.
The arrangements stated hereinabove enable this result to be achieved to some degree; the filter mass blocks a major part of the dust in suspension in the air, of course; the water vapor contained in the exhaled air, which is saturated at the temperature of the organism, condenses on the filter mass which is therefore heated substantially to the body temperature; inhaled air, arriving at the temperature of the ambient air, is warmed and takes up moisture in contact with the filter mass which is at a higher temperature and contains the condensed water.
However, these known arrangements do not provide sufficient protection against clogging of the pores of the filter mass by mucus expelled from the trachea. A defense measure of the organism against foreign bodies such as dust, microorganisms and dead cells, mucus must be blocked by the grill disposed between the surgical opening and the filter mass; however the current of exhaled air tends to entrain the mucus from the posterior side of the grill on which it is deposited towards the anterior side and from there to spread it over the surface of the filter mass in contact with the anterior side of the grill. Subsequent drying of the mucus forms a hardened crust which clogs the filter mass. The tracheotomy filter must then be replaced.